A typical gasoline engine executes air-fuel ratio feedback control to correct fuel injection amount based on a detection value from the oxygen concentration sensor located in the exhaust passage. To improve the exhaust gas property, diesel engines have been proposed that are configured to control exhaust gas recirculation amount by an exhaust gas recirculation mechanism, that is, an EGR mechanism based on the oxygen concentration of exhaust gas detected by the oxygen concentration sensor located in the exhaust passage.
However, as shown in FIG. 9, the actual output property of the oxygen concentration sensor, that is, the relationship between the oxygen concentration and a detection value C can deviate from the standard output property due to time degradation, activated state, or individual differences of sensor elements. If variation occurs in the output property of the oxygen concentration sensor as described above, the oxygen concentration in the exhaust gas cannot be detected accurately. In this case, the air-fuel ratio and the exhaust gas recirculation amount are controlled based on the detection value C of low precision.
Patent Document 1 discloses a learning device, which determines that the oxygen concentration of the ambient air in the vicinity of the oxygen concentration sensor is substantially equal to the oxygen concentration of the atmospheric air on condition that the fuel injection has been stopped for a predetermined time period, and stores the detection value C of the oxygen concentration sensor at the time as a learned value Cstd. More specifically, when it is predicted that the oxygen concentration in the vicinity of the oxygen concentration sensor will become equal to the oxygen concentration of the atmospheric air (atmospheric oxygen concentration Datm shown in FIG. 9) since the deceleration state that involves fuel cut-off operation has been continued for a predetermined time period and the air drawn into the engine is fed unchanged to the exhaust passage, the learning device stores the detection value C of the oxygen concentration sensor at the time as the learned value Cstd. Thereafter, the learning device corrects the detection value C of the oxygen concentration sensor based on the stored learned value Cstd. An example of such correction includes a method in which a detection value corresponding to the atmospheric oxygen concentration Datm according to the standard output property is previously stored as a reference value Cnrm, and the detection value C of the oxygen concentration sensor is multiplied by a correction coefficient K, which is a value Cnrm/Cstd obtained by dividing the reference value Cnrm by the learned value Cstd.
Thus, even if variation occurs in the detection value C due to, for example, time degradation and individual differences, deterioration of the exhaust gas property is inhibited by executing the learning process, in which the detection value C at the time when the ambient air in the vicinity of the oxygen concentration sensor is equal to the atmospheric state is stored as the learned value Cstd, and correcting the detection value C of the oxygen concentration sensor based on the stored learned value Cstd.
However, the time period during which the fuel cut-off operation is continued during travelling of the vehicle is relatively short. Thus, the learning process cannot be completed when the fuel cut-off operation is terminated before the ambient air in the vicinity of the oxygen concentration sensor becomes equal to the atmospheric state. Thus, the state in which the learned value Cstd cannot be obtained is continued for a long period of time in the conventional configuration, and the air-fuel ratio and the exhaust gas recirculation amount are not precisely controlled.    Patent Document 1: Japanese Laid-Open Patent Publication No. 10-212999